I've always - tsset - my dynamic panel datasets. I don't see -
xtset- in
my Version 9 xt manual, or in my Version 8 basic documentation. Is
there any practical difference between -tsset- and -xtset- besides
that
-tsset- doesn't require multiple panels and that -xtset- doesn't
require
multiple time periods?

As I understand it, there is no practical difference between -tsset-
and -xtset- currently. (If that turns out to be incorrect, I'm sure
someone else will pipe in.) -xtset- was introduced in an update to
Stata 9, hence it is not in your [XT] manual if it's an early
printing. -help xtset- will provide more information. Search -help
whatsnew- for -xtset- to can learn a little about the story behind
its introduction.